In general, a radio receiver is assumed to receive radio waves having various strengths depending on the environment. For example, the strength of a received radio wave is high if the distance to a terminal or base station serving as a communication partner is short, and decreases as the distance becomes longer.
To cope with this situation, the input portion of the radio receiver generally requires a variable gain amplifier which adjusts a radio signal received by the antenna to an optimum power level and transmits it to the next stage.
The variable gain amplifier generally determines an amplification factor to maximize the ratio of signal power to noise and distortion generated in the amplifier (to be referred to as an SNDR (Signal-to-Noise and Distortion Ratio). The SNDR is given by
                    [                  Mathematical          ⁢                                          ⁢          1                ]                                                            SNDR        =                              S            in                                              N              in                        +                                                            N                  add                                +                                  N                  dist                                            G                                                          (        1        )            where Sin is the input signal power, G is the amplification factor of the variable gain amplifier, Nin is the input noise power, Nadd is the noise generated in the amplifier, and Ndist is the distortion generated in the amplifier.
Generally, the distortion generation amount depends on the magnitude of signal power in the circuit, and increases in proportion to the power of 2 or more of the magnitude of signal power. In contrast, the magnitude of noise is independent of that of signal power.
When an input signal is small and the distortion generation amount is much smaller than thermal noise, equation (1) is approximated as
                    [                  Mathematical          ⁢                                          ⁢          2                ]                                                            SNDR        =                              S            in                                              N              in                        +                                          N                add                            G                                                          (        2        )            
From equation (2), the SNDR can be increased by setting a high amplification factor G.
When an input signal voltage is large and the distortion generation amount is much larger than noise, equation (1) is approximated as
                    [                  Mathematical          ⁢                                          ⁢          3                ]                                                            SNDR        =                              S            in                                              N              in                        +                                          N                dist                            G                                                          (        3        )            
As described above, the distortion Ndist of the amplifier is proportional to the power of 2 or more of the amplification factor G. Considering this, the SNDR can be increased by setting a low amplification factor G.
From this, the variable gain amplifier adjusts the amplification factor to be high for small input signal power, and low for large signal power. The variable gain amplifier can, therefore, transmit a signal to the next stage while keeping the SNDR high in a wide range (dynamic range) of the input signal power strength. Especially in a variable gain amplifier for a high frequency band of several hundred MHz or more, it is a common practice to widen the dynamic range by switching between a plurality of amplifiers having different amplification factors.
The first arrangement example concerning such a variable gain amplifier will be explained (see, e.g., Philip Quinlan, Patrick Crowley, Miguel Chanca, Sean Hudson, Bill Hunt, Kenneth Mulvaney, Guido Retz, Cormac E. O'Sullivan and Patrick Walsh, “A Multimode 0.3-200-kb/s Transceiver for the 433/868/915-MHz Bands in 0.25-μm CMOS”, IEEE J. Solid-State Circuits, vol. 39, pp. 2297-2310, December 2004). FIG. 14 shows the first arrangement example concerning a variable gain amplifier. In a variable gain amplifier 200 in this arrangement example, two amplifiers 41 and 42 which have active elements at their input portions and are different in gain are juxtaposed. The amplification factor of the amplifier 41 is higher than that of the amplifier 42. The output terminals of the amplifiers 41 and 42 are connected to a selector 43. The selector 43 selects either of outputs from the two amplifiers 41 and 42 so as to increase the SNDR of the amplifier, and transmits the signal to the next stage.
In an actual radio receiver, the selector 43 selects the amplifier 41 when the voltage amplitude of an input signal is small and the distortion generation amount is smaller than the thermal noise generation amount, and it selects the amplifier 42 in an opposite case.
In this arrangement example, the active element is arranged at the input portion in order to increase the maximum gain of the variable gain amplifier. As represented by equation (2), when the signal voltage is small, the SNDR becomes higher for a larger gain. That is, increasing the maximum gain means further decreasing the lower limit of the dynamic range to widen the dynamic range.
The second arrangement example concerning a variable gain amplifier will be explained (see, e.g., Japanese Patent Laid-Open No. 2006-311623). FIG. 15 shows the second arrangement example concerning a variable gain amplifier. In a variable gain amplifier 210 in this arrangement example, an amplifier 51 for amplifying an input signal, and an amplifier 52, at the input stage of which an attenuator 54 is arranged, are juxtaposed. The amplifiers 51 and 52 are equal in gain and have active elements at their input portions. The amplifiers 51 and 52 are connected to a control circuit 53. By operating either the amplifier 51 or 52, the control circuit 53 can extract only an output from one amplifier.
In this arrangement example, when the control circuit 53 selects the amplifier 51, the gain of the variable gain amplifier 210 is directly indicated by that of the amplifier 51. When the control circuit 53 selects an output from the amplifier 52, the gain of the variable gain amplifier 210 is represented by a value obtained by dividing the gain of the amplifier 52 by the attenuation factor of the attenuator 54. Similar to the first arrangement example, the second arrangement example can change the gain in two steps by switching the operation states of the amplifiers 51 and 52 by the control circuit 53.